The invention relates to electron multipliers. In particular, the invention relates to monolithic electron multipliers and microchannel plates (MCP) formed from an isotropic etchable material.
Conventional microchannel plate manufacture relies on the glass multifiber draw (GMD) process. Individual composite fibers, consisting of an etchable soluble barium borosilicate core glass and an alkali lead silicate cladding glass, are formed by drawdown of a rod-in-tube preform, packed together in a hexagonal array, and then redrawn into hexagonal multifiber bundles. These multifiber bundles are next stacked together and fused within a glass envelope to form a solid billet. The billet is then sliced, often at a small angle 8.degree.-15.degree. from the normal to the fiber axes. The resulting wafers are edged and polished into a thin plate. The soluble core glass is then removed by a suitable chemical etchant to produce a wafer containing an array of microscopic channels with channel densities of 10.sup.-5 -10.sup.7 /cm.sup.2. Further chemical treatments followed by a hydrogen reduction process produces a thin wafer of glass containing an array of hollow channels with continuous dynodes of reduced lead silicate glass (RLSG) having conductive and emissive surface properties required for electron multiplication. Metal electrodes are thereafter deposited on the faces of the wafer to complete the manufacture of a microchannel plate.
The GMD method of manufacture described, while satisfactory and economical, suffers from certain disadvantages. For example, the size of the individual channels is governed by at least two glass drawing steps in the manufacturing process. Variations in fiber diameter can cause channel diameter variation, resulting in differential signal gain, both within an MCP and from one MCP to another.
Another disadvantage of current technology concerns channel arrangement. Individual composite fibers are packed in a hexagonal array before redrawing a multifiber bundle. This local array is moderately regular, but variation of fiber size can cause some disorder, and fibers on the periphery of a drawn multifiber bundle are often disordered and dislodged. Further, when these multifibers are stacked and pressed to form a billet there are invariably disruptions in the channel array and distortions in channel cross-section at the boundaries between the multifibers. As a result of these and other processing steps, there is no long-range order in channel location, and channel geometry is not constant across the array.
The manufacture of microchannel plates according to the GMD process is also limited in the choice of materials available. The multifiber drawdown technique demands that the starting materials, namely the core and cladding, both be glasses with carefully chosen temperature-viscosity properties; the fused billet must have properties conducive to wafering and finishing; core material must be preferentially etched over the cladding with very high selectivity; the clad material must ultimately exhibit sufficient surface conductivity and secondary electron emission properties to function as a continuous dynode for electron multiplication. This set of constraints greatly limits the range of materials suitable for manufacturing MCPs with the present technology.
Multi-component alkali lead silicate and barium borosilicate glasses are typically used as the cladding and core materials, respectively, in manufacturing MCPs. To obtain satisfactory continuous dynode action with present materials, the ratio (.alpha.) of channel length (L) to channel diameter (D) is typically 40 or more. This aspect ratio is routinely achieved in conventional MCPs by virtue of the extremely high etch selectivity between core and cladding material. However, the difficulties of constructing such a substrate become more critical as the channel diameter and pitch (center to center spacing) of the channels is reduced to below 10 microns.
Attempts have been made to crystallize a photosensitive glass in a lithographically-defined pattern so as to render the crystallized regions selectively etchable from the glass leaving behind an array of channels for producing a microchannel plate. However, only moderate etch selectivity between the crystalline and glass phases yields through channels with non-parallel side walls and limits the minimum channel diameter to about 25 .mu.m. Moreover, the formation of a two-layer secondary emissive and conductive surface in the microchannels is accomplished by a number of cumbersome and difficult steps.
Attempts have also been made in selectively etching a silicon wafer sliced with a set of its crystalline (111) planes normal to the (110) faces of the slice. However, simple holes with vertical side walls extending through the wafer cannot be achieved due to well-known crystallographic constraints.